As the photoconductive material which absorbes the energy of the electromagnetic radiation such as ultraviolet rays, visible rays, infrared rays and X-rays to produce carriers for electric charge and increase the electroconductivity, there are known inorganic photoconductive materials such as Se, CdS, ZnO and As.sub.2 S.sub.3 as well as organic photoconductive materials such as poly-N-vinylcarbazole, trinitrofluorenone, phthalocyanine compounds and triphenylaminepolycarbonate. While these conventional inorganic or organic photoconductive materials are utilized in various fields depending upon their photoconductive characteristics, they have more or less some certain drawbacks; hence it is always necessary for their practical use to make any contrivance for overcoming those drawbacks. Thus, they are not satisfactory for overall purposes.
In general, organic photoconductive materials can be readily molded in a sheet or film form and easily controlled in sensitivity to the wavelength of light. Since, however, the mobility of the carrier for electric charge is small, their application in the field requiring a high speed response is restricted. On the other hand, inorganic photoconductive materials can show a high mobility of the carrier for electric charge. But, the control of their sensitivity to the wavelength of light is difficult. Even if succeeded in controlling the sensitivity, other characteristic properties such as the mobility of the carrier, the mobility and lifetime of the carrier are deteriorated, which leads to sacrifice of some of the photoconductive characteristics of the material. In order to overcome the above drawbacks inherent to organic or inorganic photoconductive materials, attempts have been made to design photoconductive materials of separation-of-function type which are constituted with organic photoconductive materials and inorganic photoconductive materials in combination. However, any satisfactory one has not been obtained yet.
One remarkable material is an amorphous silicon-hydrogen alloy (hereinafter referred to as "a-Si:H" in which "a-" stands for "amorphous") prepared by "rf" glow discharge.
This alloy has various advantages over the above conventional materials. The a-Si:H alloy has a wide sensitive wavelength region in a visible region between blue and red and good mechanical strength. Further, it is practically advantageous since it is substantially nontoxic. In applying the a-Si:H alloy as a photoconductive material, several problems should be overcome. When it is used as a photoconductive material for use in a charge-storage type device which makes use of charge depletion layers such as electrophtography or an image pick-up tube, it should have not only good photosensitivity but also electro static chargeability. The electro static chargeability of the alloy may be increased by reducing the mobility at dark of the carrier or by doping impurities to the alloy in order to shorten the lifetime of the carrier. However, as the electro static chargeability is increased, response during illumination of light and residual surface voltage are adversely affected. The impurity dopant deteriorates preferred characteristics of the a-Si:H alloy. Other method for increasing the electro static chargeability comprises providing the alloy with a blocking layer which prevents injection of the carrier from an electrode. The blocking layer is effective to a material with a low generation of the thermal carrier but may not effective to one with a high generation of the thermal carrier due to narrow band gap or shift of Fermi level to conduction band or valence band.